In many printing systems, it is common practice to develop a hardcopy of an image using a photoconductive surface. The photoconductive surface is selectively charged with a latent electrostatic image having image and background areas. A liquid developer comprising charged toner particles in a carrier liquid is brought into contact with the selectively charged photoconductive surface. The charged toner particles adhere to the image areas of the latent image while the background areas remain clean. A hardcopy material (e.g. paper) is brought directly or indirectly into contact with the photoconductive surface in order to transfer the latent image. Variations of this method utilize different ways for forming the electrostatic latent image on a photoreceptor or on a dielectric material.
Typically the liquid developer (often referred to in the art as ink or toner) comprises a thermoplastic resin (polymer) as the basis for the toner particles (also referred to as ink particles), and a non-polar liquid as a carrier liquid in which the toner particles are dispersed. Generally, the toner particles contain a colorant such as a pigment.
Two characteristics of a liquid developer are relevant to the background of the invention. These are the cohesion and the adhesion of the developer. Cohesion is the attraction exerted between the particles of the developer to keep it united throughout its mass. Adhesion is the attraction exerted between the surfaces of the toner particles after development and the surface on which they are fixed, such as the surface of the printed substrate.
Suitable additives may improve many characteristics of liquid toners. For instance, WO 96/17277 the disclosure of which is incorporated herein by reference, discloses that scuff resistance, abrasion resistance, and peel resistance of a wide class of liquid toners may be improved by the addition of a minor amount of an additional material, which at the fusing temperature used for the toner, has a much lower viscosity than the viscosity of the toner particles at the same temperature, and which forms a separate phase from the toner particles when solidified.
U.S. Pat. No. 5,055,370 discloses that a positively chargeable liquid developer having “excellent adhesiveness” may be obtained with toner particles comprising a resin which is prepared by copolymerizing monomers of three specific families; and adds that as for the copolymers containing as a copolymerizing component an acid anhydride group-containing monomer, a copolymer of styrene and maleic anhydride is preferred.
U.S. Pat. No. 5,411,834 teaches liquid developer compositions, having thermoplastic resin particles, optional pigments, a charge director, and an insoluble charge adjuvant. The charge adjuvant is comprised of a copolymer of an alkene and an unsaturated acid derivative, and the acid derivative contains pendant fluoroalkyl or pendant fluoroaryl groups. The charge adjuvant is associated with or combined with the resin and the optional pigment.
JP 1044955 teaches improving fixability of a liquid developer by dispersing in the liquid carrier a specific acrylic resin having a thermosetting functional group and a curable cross-linking agent that does not substantially react with the acrylic resin.
The disclosures of all the above-mentioned references are incorporated herein by reference.